My sister at age 6 had something that looked quite similar to what you describe, and it took ages to diagonise(sp), ended up being "Hemlock Shinline Perpura.
At one point they even told my mum she had collagens disease
i remeber that we were all out for a meal and my sister's joints were so sore she couldn't walk and had to be carried to the car, so it obviously came on really fast.
As it happens it was a very rare reaction to a very rare disease.
EXTREMLEY GLAD that your little DS is on the mend, Love to you all